From laboratory measurements, we know that a particular spectral line formed by hydrogen appears at a wavelength of 121.6 nanometers (nm). The spectrum of a particular star shows the same hydrogen line appearing at a wavelength of 121.8 nm. What is the receding speed of the star? c = 3x10^5 km/s

[negative sign denotes that it is moving away]

Explanation:

Actual wavelength of hydrogen spectral lines,

Apparent wavelength of hydrogen spectral lines from a star,

So, shift in wavelength:

Using Doppler's shift:

[negative sign denotes that it is moving away]

d) physical means can be used to separate the components of air.